For more than two decades, through two wars and domestic upheaval, the idea that al-Qaeda acted alone on 9/11 has been the basis of U.S. policy. A blue-ribbon commission concluded that Osama bin Laden had pioneered a new kind of terrorist group—combining superior technological know-how, extensive resources, and a worldwide network so well coordinated that it could carry out operations of unprecedented magnitude. This vanguard of jihad, it seemed, was the first nonstate actor that rivaled nation...